So, I made a clean install of Grav core + Admin plugin into my http://localhost/test/ which is working and showing the default page.
I copied the content of setup_subdirectory.php from the docs into setup.php i my root folder "test".
I then created test/user/sites/subsite/user/ and in there placed config, pages, plugins, themes that I copied from the base user-dir.
When I visit http://localhost/test/subsite I get a Error 404, page not found message.
Could someone point me in the right direction?
I've read all the docs about this and all forum posts that I could find about Multisite and still can't find a solution to this.